Combine all the sauce ingredient in a measuring cup and set aside.
Heat the oil in a large skillet over high heat.
In a large bowl, combine the steak strips with the cornstarch, tossing to coat. Immediately transfer the steak strips to the hot skillet and sauté until just cooked and browned in spots, 2 to 3 minutes. Transfer to a plate.
Lower the heat to medium, then pour the sauce into the skillet and bring to a simmer. Cook for 1 to 2 minutes to allow the sauce to thicken, then stir in the steak and toss to coat with the sauce.
Serve immediately, sprinkled with sesame seeds and green onions, and accompanied with steamed veggies and rice.